Sauté thinly sliced onions in a sauce pan in the butter (medium heat) until translucent, about 3 minutes. Add the sliced mushrooms and sauté with the onions for 5 minutes.
Sprinkle the flour over the cooked mushrooms and stir to combine for 1 minute.
Add heated chicken broth (or beef broth for darker soup), bring to a boil, reduce the heat to simmer - total of about 3 minutes.
Heavy cream is added and heat reduced to low.
Mix egg yolks with the sherry and sour cream - stir to combine. Temper this mixture with some of the hot soup (tablespoon at a time), whisking constantly. Transfer the tempered mixture into the soup and heat for 3 minutes.
Salt and pepper to your taste. Remove 25% of the mushroom and set aside. Blend the rest of the soup in a Vitamix or use an emersion blender until smooth. Add back the reserved mushrooms, garnish and serve hot.
A more traditional German mushroom soup would use beef broth instead of chicken and may be garnished with bacon bits.